1. Field of the Invention
The present invention relates to a single-chip microprocessor comprising a built-in self-testing function whereby said single-chip microprocessor tests whether the built-in internal circuits of the single-chip microprocessor are normal or defective.
2. Description of the Prior Art
In a conventional single-chip microprocessor with a built-in self-testing function, the test for determining whether the processor is functioning normally or not can only be used for testing the single-chip microprocessor using a tester before mounting the single-chip microprocessor to the circuit board, even though the self-testing function is built in to the microprocessor. Once the single-chip microprocessor is mounted, abnormal operation must first be detected in the system of the board to which the microprocessor is mounted in order to confirm errors or defective operation in the microprocessor.
FIG. 19 is a block diagram of the external output circuit of a conventional single-chip microprocessor. As shown in FIG. 19, the output signal from the function module 110 passes through invertor 111 directly into the external output buffer comprising p-channel MOS-FET 24 and n-channel MOS-FET 25 from which the signal is output through the external output terminal.
In a conventional single-chip microprocessor thus comprised, however, if the external output signal of the single-chip microprocessor is tested using the idle time of an operating single-chip microprocessor mounted to a circuit board, i.e., using the time in which the board-mounted single-chip microprocessor does not need to interface with other external devices during microprocessor operation, a signal change is applied to the external electronic device(s) connected to the external output terminal. As a result, self-testing cannot be executed even during the idle time of the single-chip microprocessor; determining the interrelationship between various signals with an added time factor, and similarly precise testing of board-mounted single-chip microprocessors is not possible; detecting the breakdown of or defects in single-chip microprocessors after board mounting is difficult; and it is therefore difficult and time-consuming to precisely determine both the location and the cause of errors or defects in a single-chip microprocessor.